I have a bar-end mirror on the bike, but also have a helmet mounted mirror - I use both. The helmet-mounted mirror doesn't give me the wide field of the mirror on the bars, but it is useful in that I can scan around with it just by turning my head - very useful for the cursory look. If I'm about to execute a turn, or change direction, I'll take a quick glance over the shoulder just to make sure. I don't rely on the mirrors entirely, but I've found that I've got so used to them, that I'd feel naked without them.
- Wil